1. Field of the Invention
The present invention relates to an EL panel in which an EL element formed on a substrate is sealed between the substrate and a cover member. Further, the present invention relates to an EL module in which an IC is mounted in the EL panel. Incidentally, in the present specification, the EL panel and the EL module are generally referred to as a light-emitting device. The present invention further relates to an electronic instrument using the light-emitting device.
2. Description of the Related Art
In recent years, a technique for forming a TFT on a substrate has been greatly advanced, and application to an active matrix display device has been advanced. Especially, since a TFT using a polysilicon film has an electron field-effect mobility (also called mobility) higher than that of a TFT using a conventional amorphous silicon film, a high speed operation is possible. Thus, control of a pixel, which is conventionally performed by a driving circuit outside a substrate, can be performed by a driving circuit formed on the same substrate as the pixel.
In this sort of active matrix display device, various merits, such as reduction of manufacturing costs, miniaturization of an electro-optic device, improvement of a yield, and reduction of a throughput, can be obtained by forming various circuits and elements on the same substrate.
Further, research of an active matrix type light-emitting device including an EL element as a self-luminous element has been actively carried out. The light-emitting device (EL display) including the EL element is also called an organic EL display (OELD: Organic EL Display) or an organic light-emitting diode (OLED: Organic Light-emitting Diode).
The light-emitting device is of a self-luminous type differently from a liquid crystal display device. The EL element has such a structure that a layer (hereinafter referred to as an EL layer) containing an organic compound is sandwiched between a pair of electrodes (anode and cathode), and the EL layer has normally a laminate structure. Typically, there is cited a laminate structure “hole transporting layer/light-emitting layer/electron transporting layer” proposed by Tang et al. of Eastman Kodak Company. This structure has a very high luminous efficiency, and most of the light-emitting devices on which research and development has been made at present adopt this structure.
In the EL element, luminescence (Electro Luminescence) generated by application of an electric field is obtained, and it includes an anode layer, an EL layer, and a cathode layer. Luminescence in an organic compound includes light emission (fluorescence) generated when a single excited state returns to a ground state and light emission (phosphorescence) generated when a triplet excited state returns to the ground state, and the EL display of the present invention may use either light emission.
In addition, there may be also adopted a structure in which laminating is made on an anode in the order of a hole injecting layer/a hole transporting layer/a light-emitting layer/an electron transporting layer or a hole injecting layer/a hole transporting layer/a light-emitting Layer/an electron transporting layer/an electron injecting layer. The light-emitting layer may be doped with a fluorescent pigment or the like.
In the present specification, all layers provided between a cathode and an anode are generally referred to as an EL layer. Thus, all of the foregoing hole injecting layer, hole transporting layer, light-emitting layer, electron transporting layer, electron injecting layer, and the like are included in the EL layer.
Besides, in the present specification, an element formed of an anode, an EL layer and a cathode is referred to as an EL element.
In a light-emitting device, a plurality of pixels are provided in a matrix form, and each of the plurality of pixels includes a thin film transistor (TFT) and an EL element. FIG. 4 is a circuit diagram of a pixel of a general light-emitting device. A pixel 400 includes a switching TFT 401, a current controlling TFT 402, an EL element 403, a source signal line 404, a gate signal line 405, a power supply line 406, and a capacitor 407.
A gate electrode of the switching TFT 401 is connected to the gate signal line 405. One of a source region and a drain region of the switching TFT 401 is connected to the source signal line, and the other is connected to a gate electrode of the current controlling TFT 402. A source region of the current controlling TFT 402 is connected to the power supply line 406, and a drain region is connected to an anode or a cathode of the EL element 403.
In the case where the anode of the EL element 403 is connected to the drain region of the current controlling TFT 402, the anode of the EL element 403 becomes a pixel electrode, and the cathode becomes a counter electrode. On the contrary, in the case where the cathode of the EL element 403 is connected to the drain region of the current controlling TFT 402, the anode of the EL element 403 becomes the counter electrode, and the cathode becomes the pixel electrode.
Note that, in the present specification, a potential difference between a potential of a pixel electrode and a potential of a counter electrode is called an EL driving voltage, and this EL driving voltage is applied to the EL layer.
Note that, as shown in FIG. 4, the capacitor 407 is provided to be connected to the current controlling TFT 402 and the power supply line 406.
The potential (power source potential) of the power supply line 406 is kept constant. The potential of the counter electrode of the EL element 403 is also kept constant. The potential of the counter electrode has a potential difference from the power source potential to such a degree that the EL element emits light when the power source potential is applied to the pixel electrode of the EL element.
The switching TFT 401 comes to have an on state by a selection signal inputted to the gate signal line 405. Incidentally, in the present specification, that a TFT comes to have an on state means that a drain current of the TFT comes to have a state of more than 0.
When the switching TFT 401 comes to have the on state, a video signal inputted from the source signal line 404 is inputted to the gate electrode of the current controlling TFT 402 through the switching TFT 401. Incidentally, in the present specification, the video signal means an analog signal including image information. Incidentally, that a signal is inputted to the gate electrode of the current controlling TFT 402 through the switching TFT 401 means that a carrier moves through an active layer of the switching TFT 401, and a potential of a video signal is given to the gate electrode of the current controlling TFT 402.
The amount of current flowing through the channel formation region of the current controlling TFT 402 is controlled by a gate voltage Vgs of a potential difference between the gate electrode and the source region of the current controlling TFT 402. Thus, the potential given to the pixel electrode of the EL element 403 is determined by the height of the potential of the video signal inputted to the gate electrode of the current controlling TFT 402. The emission luminance of the EL element (luminance of light emitted from the EL element) is controlled by the height of the potential given to the pixel electrode. That is, the luminance of the EL element 403 is controlled by the potential of the video signal inputted to the source signal line 404 and a gradation display is carried out.
FIG. 5 shows the relation between the emission luminance (cd/m2) of an EL element and the current density (mA/cm2). The relation between the emission luminance of the EL element and the current density is linear. That is, when the current density of the EL element becomes high at a constant rate, the emission luminance of the EL element also becomes high at a constant rate. The current density is determined by a drain current Id of the current controlling TFT 402.
Although it is desirable that TFTs formed in a pixel portion of a light-emitting device have the same characteristics, actually, the characteristics of the respective TFTs are subtly different from one another. Particularly, threshold Vth of a TFT is influenced by a difference in crystallinity of an active layer, an impurity unintentionally mixed in the active layer, and the like. Thus, there has been a case where Vth is different among the TFTs. Incidentally, in the present specification, the active layer means a semiconductor film including a source region, a drain region, and a channel forming region of a TFT.
When the value of the threshold Vth of the TFT becomes different, the value of the drain current Id also becomes different. Expression 1 indicates the relation between the drain current Id and the threshold Vth.
                    Id        =                              1            2                    *          μ          *                      C            u                    *                      W            L                    *                                    (                              Vgs                -                Vth                            )                        2                                              [                  Expression          ⁢                                          ⁢          1                ]            
Where, μ (m2/V·sec) indicates a mobility of the TFT, and Co (F/cm2) indicates a capacitance value per unit area of a capacitance (gate capacitance) formed by a gate electrode, an active layer and a gate insulating film of the TFT.
Besides, W and L indicate a channel width and a channel length of a channel forming region of the TFT, respectively, and its position is shown in FIG. 6. FIG. 6 is a view schematically showing the TFT, and the active layer includes a channel forming region 601, a source region 602, and a drain region 603. The channel forming region 601 is provided to be sandwiched between the source region 602 and the drain region 603. Although not shown in FIG. 6, there is also a case where an LDD region is provided between the channel forming region 601 and the source region 602 or the drain region 603.
A gate electrode 604 is provided over the channel forming region 601 through a gate insulating film (not shown). Note that in the present specification, the channel forming region 601 is included in a portion of an active layer 600 overlapping with the gate electrode 604 and indicates a portion where a channel is actually formed when a voltage is applied to the gate electrode 604.
The channel length L is a length of the channel forming region in the direction in which a carrier of a free electron or free hole flows. The channel width W is a length of the channel forming region in the direction vertical to the direction in which the carrier flows. Although the TFT shown in FIG. 6 has a single gate structure, in the case of a TFT having a multigate structure such as a double gate structure or a triple gate structure, the channel length L is defined as the sum of lengths of channel forming regions formed under all gate electrodes in the direction in which the carrier flows.
As indicated by the expression 1, when the value of the threshold voltage Vth is varied, the value of the drain current Id is also varied. Thus, if the value of the threshold voltage Vth of the current controlling TFT is different among pixels, even if video signals having the same potential are inputted to the respective pixels, the emission luminance of the EL element becomes different among the pixels. Note that in the present specification, to input a signal to a pixel means to input a signal to a gate electrode of a current controlling TFT through a switching TFT included in the pixel.
If the emission luminance is not uniform in all pixels of the light-emitting device, unevenness of luminance (uneven luminance) appears in an image displayed on the pixel portion and is visually recognized by an observer.
In order to suppress the foregoing uneven luminance, as shown in FIG. 18, a light-emitting device having a structure in which four TFTs are provided in a pixel is devised (SID'98 DIGEST 4.2 “Design of an Improved Pixel for a Polysilicon Active-Matrix Organic LED Display” R. M. A. Dawson etc.).
In FIG. 18, reference numeral 1701 designates a first thin film transistor; 1702, a second thin film transistor; 1703, a third thin film transistor; and 1704, a fourth thin film transistor. The emission luminance of an EL element 1705 is controlled by the first to fourth four thin film transistors.
When the first thin film transistor 1701 comes to have the on state by a selection signal inputted to a gate signal line (G), and the third thin film transistor 1703 comes to have the on state by a signal inputted to a first signal line (AZ), a gate electrode and a drain region of the second thin film transistor 1702 are short-circuited. Since the fourth thin film transistor 1704 is in an off state by a signal inputted to a second signal line (AZB), a gate voltage Vgs of a voltage between the gate electrode and a source region of the second thin film transistor 1702 enters into a subthreshold region determined by a leak current.
Next, the third thin film transistor 1703 comes to have the off state by a signal inputted to the first signal line (AZ). Then, a video signal is inputted to a source signal line (S) and a potential of the video signal is given to the gate electrode of the second thin film transistor 1702 through the first thin film transistor 1701 having the on state. Accordingly, the gate voltage Vgs of the third thin film transistor 1703 becomes a potential obtained by adding the potential of the video signal to the gate voltage Vgs having entered into the subthreshold region.
Next, the first thin film transistor 1701 conies to have the off state by a selection signal inputted to the gate signal line (G). Then, the fourth thin film transistor 1704 comes to have the on state by a signal inputted to the second signal line (AZB). Since a current flowing through the channel forming region of the TFT depends on the value of the gate voltage Vgs of the third thin film transistor 1703, the current having the intensity corresponding to the potential of the video signal is inputted to a pixel electrode of the EL element 1705.
In the case of the light-emitting device having the above structure, in the case where video signals having the same potential are inputted to the source signal line, it is possible to prevent the potential given to the pixel electrode from being varied by the value of the threshold Vth of the second thin film transistor 1702. Thus, the uneven luminance of an image can be suppressed. However, if the number of thin film transistors provided in each pixel is increased, the opening ratio is lowered, and it becomes necessary to increase a current flowing through an EL element in order to obtain constant luminance. If the current flowing through the EL element is increased, deterioration of the EL layer is accelerated, which is not preferable.
Besides, if the number of TFTs provided in a pixel is increased, there is a fear that yield of the light-emitting device itself is lowered.